There are several experimental setups involving rotating machinery that require some form of synchronization. Furthermore, many of these setups require continuous tracking of the revolution period to yield an accurate pulse(s)-per-revolution signal generator. As an example, optical experiments that use luminescent coatings on rotating equipment require a phase delay generator (PDG) to control data acquisition. This PDG generally uses a once-per-revolution input signal and computes the time delay necessary to advance the rotor to a user selectable fraction of a revolution in degrees. A continuously updated calculation of delay time is performed using the period from the current revolution while the actual delayed output is executed on the following revolution. Ultimately, control of the rotational timing allows the acquisition of rotor data in the same relative location regardless of speed. This is critical to the acquisition of radiometric luminescent coating data which relies on normalizing the data at the same rotational location.
Acquiring data using the same image view while maintaining the same number of integrated illumination pulses produces an image pair of the luminescent coating with intensity differences corresponding to changes in pressure or temperature. Therefore, an integrated pulse count, which allows an exact number of integrated flash illuminated rotations, is also an essential feature for a PDG. Sometimes, the output pulse width factors into the integration time, and the output pulse width must be controlled to meet test requirements.
Furthermore, possessing two channels with separate once-per-revolution input signals may allow the rotational control of two independent rotors. A PDG with this kind of independent control would enable rotor interactions to be captured with standard commercial off-the-shelf (COTS) imaging devices.
Other experimental setups such as a particle image velocimetry (PIV) system use a pulsed laser that must be synchronized to the rotating blades of a turbine. When acquiring PIV data from rotating machinery, a typical setup also uses a once-per-revolution signal from the rig as a trigger. Afterward, a delay generator is used to phase step the actual laser pulse to the blade passage of interest in the machine. The phase delay can be a significant fraction of the rig single rotation time, but the assumption in this configuration is that the rig is running at constant speed and the phase delay will always locate the blade of interest at the exact position in the field of view in the PIV system. However, if the rotation speed changes, the fixed phase delay is no longer accurate. Using a PDG that tracks the revolution period keeps the PIV system tuned to the true rotational speed of the rig, as in the case of the luminescent coating rig mentioned earlier. The constant correction for rotational speed yields a repeatable blade location for acquiring the PIV image data. A fixed phase delay is not required as in older PIV systems, since the user can specify a circumferential rotational angle and the PDG computes the correct delay to position the desired blade at the desired circumferential position.
However, there are PIV applications where a once-per-revolution signal is provided, but a once-per-blade signal is required. For example, one particular PIV system configuration uses a pulsed laser and a camera with an electromechanical shutter to acquire data. This system possesses a shutter that requires 24 ms to open and then close and a laser that must be triggered every 100 ms in order to remain at thermal equilibrium. For a 6000 rpm machine with 12 blades, the once-per-revolution time is 10 ms. Using the once-per-revolution signal in tandem with a 100 ms delay circuit that ignores input signals during its delay cycle would fire the laser anywhere from 100 ms to 110 ms, which is not sufficient to keep the laser running at a stable 10 Hz. Accordingly, an improved pulse generator that can generate once-per-blade signals would reduce the laser firing range to 100 ms to 100.8 ms for the 12 blade case which may be beneficial for timing stability.